A good solution for await in try/catch/finally?
Solution 1
You should know that since C# 6.0, it's possible to use await
in catch
and finally
blocks, so you could in fact do this:
try
{
// Do something
}
catch (Exception ex)
{
await DoCleanupAsync();
throw;
}
The new C# 6.0 features, including the one I just mentioned are listed here or as a video here.
Solution 2
If you need to use async
error handlers, I'd recommend something like this:
Exception exception = null;
try
{
...
}
catch (Exception ex)
{
exception = ex;
}
if (exception != null)
{
...
}
The problem with synchronously blocking on async
code (regardless of what thread it's running on) is that you're synchronously blocking. In most scenarios, it's better to use await
.
Update: Since you need to rethrow, you can use ExceptionDispatchInfo
.
Solution 3
We extracted hvd's great answer to the following reusable utility class in our project:
public static class TryWithAwaitInCatch
{
public static async Task ExecuteAndHandleErrorAsync(Func<Task> actionAsync,
Func<Exception, Task<bool>> errorHandlerAsync)
{
ExceptionDispatchInfo capturedException = null;
try
{
await actionAsync().ConfigureAwait(false);
}
catch (Exception ex)
{
capturedException = ExceptionDispatchInfo.Capture(ex);
}
if (capturedException != null)
{
bool needsThrow = await errorHandlerAsync(capturedException.SourceException).ConfigureAwait(false);
if (needsThrow)
{
capturedException.Throw();
}
}
}
}
One would use it as follows:
public async Task OnDoSomething()
{
await TryWithAwaitInCatch.ExecuteAndHandleErrorAsync(
async () => await DoSomethingAsync(),
async (ex) => { await ShowMessageAsync("Error: " + ex.Message); return false; }
);
}
Feel free to improve the naming, we kept it intentionally verbose. Note that there is no need to capture the context inside the wrapper as it is already captured in the call site, hence ConfigureAwait(false)
.
user2397050
Updated on January 17, 2020Comments
-
user2397050 over 4 years
I need to call an
async
method in acatch
block before throwing again the exception (with its stack trace) like this :try { // Do something } catch { // <- Clean things here with async methods throw; }
But unfortunately you can't use
await
in acatch
orfinally
block. I learned it's because the compiler doesn't have any way to go back in acatch
block to execute what is after yourawait
instruction or something like that...I tried to use
Task.Wait()
to replaceawait
and I got a deadlock. I searched on the Web how I could avoid this and found this site.Since I can't change the
async
methods nor do I know if they useConfigureAwait(false)
, I created these methods which take aFunc<Task>
that starts an async method once we are on a different thread (to avoid a deadlock) and waits for its completion:public static void AwaitTaskSync(Func<Task> action) { Task.Run(async () => await action().ConfigureAwait(false)).Wait(); } public static TResult AwaitTaskSync<TResult>(Func<Task<TResult>> action) { return Task.Run(async () => await action().ConfigureAwait(false)).Result; } public static void AwaitSync(Func<IAsyncAction> action) { AwaitTaskSync(() => action().AsTask()); } public static TResult AwaitSync<TResult>(Func<IAsyncOperation<TResult>> action) { return AwaitTaskSync(() => action().AsTask()); }
So my questions is: Do you think this code is okay?
Of course, if you have some enhancements or know a better approach, I'm listening! :)
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user2397050 almost 11 yearsThank you but unfortunately I know this method already. That's what I do normally, but I can't do this here. If I simply use
throw exception;
in theif
statement the stack trace will be lost. -
DavidRR almost 8 yearsSupport for await in catch/finally blocks in C# 6.0 is also indicated on Wikipedia.
-
Sam Hobbs about 7 years@DavidRR. the Wikipedia is not authoritive. It is just another web site among millions as far as this is concerned.
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julealgon over 5 yearsWhile this is valid for C# 6, the question was tagged C# 5 from the very beginning. This makes me wonder if having this answer here is confusing, or if we should just remove the specific version tag in these cases.